Current data storage on a network is oftentimes arranged in what is known as a Network Attached System (NAS) in which a plurality of clients, for example, user terminals such as user computers, are connected to a network to a server or storage system which either has storage arrays built into the storage system, or are somehow connected to cabinets containing storage arrays. Examples of such servers might be a server such as is available from Sun Microsystems connected to a cabinet composed of a storage array such as those available under the names Symmetrix or Clariion available from EMC Corporation.
An alternative to such a server/storage combination would be a combined unit which includes storage array and front end host intelligence as a single package such as is available from EMC Corporation under the identifier IP4700. For the sake of consistency, all of these types of systems will interchangeably be hereafter referred to as a “host storage system.” Such a system combines block storage and file protocols in one. Examples of network protocols employed are those which are readily known to those of ordinary skill in the art as NFS, CIFS (SMB), FTP, etc.
In such networks, a number of clients are connected on the network and actively access files, read them, write to them, create them, delete them, and perform other operations on the files in storage.
As previously discussed, the clients might be personal computers or stand-alone terminals, or other like systems having intelligence in order to operate on the client side of the protocols. The network can be a Ethernet-type network and can have switches. Similarly, it could be a fibre channel-type environment, i.e., anything that runs the network protocol on a fibre channel environment, i.e., IP (Internet Protocol) over fibre, is another environment alternative of how Network Attached Storage is implemented.
It is often the case that it is desirable to replace existing host storage systems for a number of reasons. For example, a system may become out of date and the network users may want to upgrade the system. A problem with replacing such a system is that it is undesirable to disrupt client access to the data. If the system desired to be replaced is disconnected from the network, then data, files and directories transferred from that system to the new system disconnected from the network, then client access is interrupted. Furthermore, the means of copying the data to the new system may not accurately preserve all file and filesystem attributes. For example, Windows/CIFS based tools will frequently not preserve file hard link attributes, while Unix/NFS based tools will typically not preserve ACL (Access Control List) attributes.
Currently, one way the data migration is done by taking the original host storage system off line. The data on the host storage system is moved to tape, and then copied onto the replacement host storage system. Alternatively, the replacement system can be connected directly to the network and the data could be copied over the network, but access to the data, files and directories is disabled for periods of up to several days. In addition, if the data migration fails in the middle of the operation, the migration has to restarted.
An alternative way of doing migration is to allow the clients to continue to access the original host storage system while copying to the replacement host storage system occurs. The problem with such a migration is that a copy is kept on the original system while trying to bring over all of the data. After the migration is completed, the two host systems have to be taken off the network for a final sweep to verify that all the data has been copied, which results in the system having to be taken off line.
These and other problems associated with migrating data, files and directories from one host storage system to another host storage system are avoided in accordance with the methods and systems described herein.